JP, go to the local tip, get a side panel off an old fridge or something, get a pair of tin snips and get cutting. Fold an edge and drill a hole to match your number plate bolt hole and bolt it on. If you want to be fancy make a template out of cardboard first! And don't forget to paint it black. Blue book will tell you max and min heights it should be. If you want to save weight then make out of ali sheet.
Worst case scenario is to use a ruler in a fold of carboard and lots of tape but maybe that is a bit Blue Peter'ish!
Malcolm